fix(Migrator): harden DDL matching and error reporting

[h2zi]

A batch of small, related fixes to how the migrator matches DDL and reports failures. Each is independently small; grouped because they share the theme (and the tests).

• GetTables: exclude sqlite_* internal tables — sqlite_sequence appeared in the list as soon as any table used AUTOINCREMENT.
• HasConstraint: match the exact constraint name via the parsed DDL and the existing compileConstraintRegexp instead of LIKE substrings, which matched prefixes (age_pos matched age_positive).
• DropColumn: report an error when the column is not present in the DDL — removeColumn returned false silently and the table was rebuilt unchanged, so callers saw success without any effect.
• removeColumn: match unquoted column names — the regexp required a quote or space before the name, so columns of hand-written DDL like CREATE TABLE t (id integer, name text) were never matched (same silent no-op as above).
• getRawDDL: report table not found instead of passing an empty string to parseDDL, which surfaced as a confusing invalid DDL.
• uniqueRegexp: recognize lowercase / no-space table-level unique(...) constraints; every other DDL regexp is already case-insensitive. Written to not mistake a column named uniqueid for a constraint (requires the parenthesized column list).

Tests: TestGetTablesExcludesInternal, TestConstraintAndColumnMatching, TestParseDDL_LowercaseUnique.

No dependency on the other open PRs — merges cleanly in any order.

🤖 Generated with Claude Code

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Merge order (this batch: #241 → #242 → #243 → #244 → #245 → #246, plus #234 from the previous batch): all seven are functionally independent and merge cleanly in any order. Several append tests to the same test files, so whichever merges later may show a trivial append-only conflict in migrator_test.go/ddlmod_test.go — I'll rebase the remaining ones promptly after each merge, as before.

[Copilot]

Pull request overview

This PR hardens the SQLite migrator’s DDL parsing/matching logic to avoid false positives and silent no-ops, and improves error reporting around missing tables/columns.

Changes:

• Filter GetTables() results to exclude sqlite_* internal tables.
• Rework HasConstraint() to parse table DDL and match exact constraint names; make DropColumn() error when a column is not found in the parsed DDL; improve getRawDDL() “table not found” reporting.
• Extend DDL parsing to recognize lowercase / no-space table-level unique(...) constraints; loosen column-removal matching to handle unquoted column definitions.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
migrator.go	Updates table listing, constraint detection, drop-column behavior, and DDL retrieval error handling.
migrator_test.go	Adds tests covering internal-table exclusion, constraint-name matching, and drop-column failure modes.
ddlmod.go	Adjusts DDL regexes for UNIQUE parsing and improves column removal matching for unquoted DDL.
ddlmod_test.go	Adds a regression test for lowercase table-level unique(...) parsing.

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